1. Field of the Invention
This invention is directed to an electrical switching apparatus and, more particularly, to a circuit interrupter, such as a circuit breaker, including a trip mechanism, and, most particularly, to a circuit breaker including an electromagnetic trip actuator.
2. Background Information
Electrical switching apparatus include, for example, circuit switching devices and circuit interrupters such as circuit breakers, contactors, motor starters, motor controllers and other load controllers. Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. Nos. 4,528,531; 4,606,313; 4,887,057; 5,200,724; and 5,341,191. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition.
Molded case circuit breakers include a pair of separable contacts per phase which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. Typically, such circuit breakers include an operating mechanism, which is designed to rapidly open and close the separable contacts, and a trip mechanism, which senses overcurrent conditions in an automatic mode of operation. The trip mechanism causes a trigger mechanism or latch to release the operating mechanism thereby tripping open the separable contacts.
The function of a circuit breaker is to force the current in an electrical circuit to zero magnitude. In an electromechanical circuit breaker, for example, an electromagnetic trip actuator converts the electrical circuit current to a magnetic field and, hence, to a mechanical force. At a particular level of current, the trip actuator causes the trigger mechanism or latch to release the operating mechanism and trip open the separable contacts. The speed of this release action is key to successful and safe circuit interruption. With a faster release, the quantity of electrical energy that is seen by the electrical circuit and its components is reduced, thereby increasing the useful life of the circuit breaker.
Some types of circuit breakers include an electronic trip unit for automatically interrupting the current flow. Still other types of circuit breakers include an analog trip unit to automatically interrupt current flow.
Further types of circuit breakers employ a blow-open feature to assist the opening of the separable contacts, as there is an inherent time delay in the response of the trip mechanism and the operating mechanism to overcurrent conditions. The blow-open feature, in response to the very high magnetic repulsion forces generated by short circuit current flowing through the circuit breaker, permits a moveable contact arm to rotate independently of a carrier assembly of the operating mechanism. A slot motor, for example, may be employed to concentrate the magnetic field generated upon a relatively high level short circuit or fault condition to increase the magnetic repulsion forces between a rigid conductor on which a main contact is securely fastened and the movable contact arm. This rapidly accelerates the separation of the contacts and results in a relatively high arc resistance and, hence, limits the magnitude of the fault current.
In a direct current (DC) system, it is necessary to stretch the arc across the separable contacts to a point of instability by building up arc voltage via arc resistance. As the arc collapses, the cessation of the electrical circuit current begins. The gap or space between the parting separable contacts is essential to the process of interrupting a DC current. Likewise, the speed of parting of the separable contacts adds directly to the efficiency of the circuit interruption operation.
A somewhat similar situation occurs with alternating current (AC), except that there is a natural point of zero current at the end of the AC current pulse. The current limiting action is such as to force the current to zero magnitude ahead of the natural AC current pulse zero crossing. Again, the speed of the trip mechanism dictates the circuit interruption efficiency.
Some circuit breakers employ solid state sensors to detect the magnitude of the electrical circuit current for the purpose of trip control and other time related operations. These sensors are based on current pulse times and commonly utilize current transformers (CTs) which have polarity memories. The most recent electrical current being sensed by the CT often has to repolarize the CT's magnetic circuit before the sensed current signal's orientation is correct to permit the CT to begin proper sensing for the purpose of timing and trip initiation. Accordingly, the CT system is commonly "slow" to initiate the trip function in its own right. Typically, "slow" is in the general order of a 1/2 cycle delay (e.g., 8.3 ms in a 60 Hz circuit). Accordingly, there is room for improvement in the circuit interruption function of electrical switching apparatus.